For $9.99, SoundAMP does the work of hearing aids costing hundreds of dollars.

An iPhone can take the place of all kinds of gear--an air mouse, a metronome, a golfer's GPS, and so on. Now, thanks to SoundAMP, it can fill in for a hearing aid as well.

The $9.99 app turns an iPhone or second-gen iPod Touch into an "interactive hearing device"--a fancy way of saying it amplifies the sound around you.

All you do is fire up SoundAMP and plug in a pair of earphones. (iPod Touch users will need one with an inline microphone.) Then just raise or lower the oversize slider until you get the volume you want.

The app also lets you adjust the tone to your liking, helpful for users who have hearing loss in specific tonal areas.

There's even a kind of "audio DVR" feature: you can replay the last 5 or 30 seconds of a conversation just by tapping an onscreen button.

According to the developers, SoundAMP improves your hearing quality in a variety of environments, including lecture halls and noisy restaurants. Thus, it has the potential to help students as well as the hearing-impaired.

At $9.99, the app is considerably cheaper than any hearing aid--though obviously you need a not-so-cheap iPhone or iPod Touch to use it. If you happen to be among the target audience for this app, hit the comments and let us know if it (sorry) sounds like a winner.